The invention relates to methods for fabricating fluid injection devices, and more particularly, to methods for fabricating fluid injection devices comprising a passivation layer with a substantially planar surface.
Typically, fluid injectors are employed in inkjet printers, fuel injectors, biomedical chips and other devices. Among inkjet printers presently known and used, injection by thermally driven bubbles has been most successful due to its reliability, simplicity and relatively low cost.
FIG. 1 is a cross section of a conventional monolithic fluid injector 1 disclosed in U.S. Pat. No. 6,102,530, the entirety of which is hereby incorporated by reference. A structural layer 12 is formed on a silicon substrate 10. A fluid chamber 14 is formed between the silicon substrate 10 and the structural layer 12 to receive fluid 26. A first heater 20 and a second heater 22 are disposed on the structural layer 12. The first heater 20 generates a first bubble 30 in the chamber 14, and the second heater 22 generates a second bubble 32 in the chamber 14 to inject the fluid 26 from the chamber 14.
Conventional monolithic fluid injectors using a bubble as a virtual valve are advantageous due to reliability, high performance, high nozzle density and low heat loss. As inkjet chambers are integrated in a monolithic silicon wafer and arranged in a tight array for high device spatial resolution, no additional nozzle plate is required for assembly.
The structural layer 12 of the conventional monolithic fluid injector 1 comprises low stress silicon nitride. The lifetime of the injector 1 is, however, determined by thickness of the structural layer. Moreover, a droplet may deviate from the desired direction due to structural layer insufficient thickness. Additionally, since heaters 21 and 22 are located on the structural layer, the heat generated by the heaters 22 and 23 may pass through the structural layer into the chamber, causing crosstalk and disturbing the operating frequency.
It is therefore important to provide a fluid injector capable of effectively dissipating heat and having a strengthened structural layer. A metal layer on the structural layer conducts and dissipates residual heat effectively and strengthens the structural layer. The conventional metal layer can be made of gold, platinum, nickel, or nickel based alloy deposited by electrical plating. An under bump metal (UBM) layer is formed before the metal layer is plated. The surface of the metal layer can, however, be roughened after the UBM layer is removed. The rough surface of the metal layer can, however, cause fluid residue causing the trajectory of droplet flight to deviate.